Re: Stan White Powrtron amplifier:well suited to modern speakers?

On Wed, 11 Jun 2008 10:31:27 +0100, "Serge Auckland"
<sergeauckland@xxxxxxxxxxxxxx> wrote:

I don't know about Power Feedback, but some amplifiers in the '50s/ early
60s had adjustable positive feedback as well as by-then conventional
negative feedback. Pye's professional monitoring amplifiers had this in
particular.

The idea is to create a negative output impedance (the voltage rises with
increasing load, rather than drops as is usual)
to offset the highish output impedance of valve amplifiers. I incorporated
this into a valve amplifier I designed in the '70s, and it worked a treat. I
could adjust the output impedance from a few ohms positive to negative. If
the sense point was brought out to a terminal, and a sense wire was added to
the 'speaker cables, you could ensure a zero output impedance actually
across your loudspeakers after a length of 'speaker cable.

I had no problems with the positive/negative feedback system, and I wonder
why it wasn't more common, possibly because it needed to be adjusted for
each amplifier and so add to manufacturing cost.

Maybe a better description of the "Powertron" and such was that
output current was sensed in addition to output voltage. Some
magic elixir was then dialed in.

It seems strange to us, a generation after elegant algebraic
models of loudspeaker response were made, but in those Dark
Ages, before modern analogies to LC filters were available,
an awful lot of *literal* cut-'n-tryin' was the order of the
day.

My favorite example from that era is the Klipsch model "Cornwall",
surely dated now, but still loved in some circles. Paul designed
the ported box entirely by cutting wood, assembling a box, and
measuring in the best way available at the time (and this was before
they had their bigger anechoic room) and iterating.

Decades later, in the 1970's, when better instrumentation was
possible, the cut-'n-try final version could be measured to be
a durned-near-enough-to a QB3 alignment.



These archaic things seem foolish, like wondering how gravity
works, until we remember that *we* don't know how gravity
works either. Life's always a work in progress. Science is
the same thing, except without the verneer of belief.

Much thanks, as always,

Chris Hornbeck
"True, but only for large values of zero."
-Mike Rivers
.

Relevant Pages

  • Re: Stan White Powrtron amplifier:well suited to modern speakers?
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